Errors creating .stl file
In the blend file from the link above the main reason for problems with 3D printing is going to be standard cases of non-manifold geometry.

There are a lot of areas with overlapping geometry with multiple vertices in the same place:



These will cause errors because of interesecting geometry. It can be previewed with enabling Intersect geometry for real-time preview in the Properties shelf > Mesh Analysis rollout (available in Edit mode):



As there are many places with such tiny errors it's easier to fix them all at once; first off in Edit mode with all selected use Remove Doubles command to get rid of duplicated vertices. Note to set threshsold quite small, like 0,000001 as otherwise there will be some changes in the letters form (tiny changes though).

After ~1200 vertces removed run Ctrl+Alt+Shift+M or Select > Select All By Trait > Non-Manifold from 3D viewport header. There will be some vertices selected in the area in '8' digit:



There could be some overlapping faces left next to faces with inconsistent normals. Check if there are and remove doubles once again if needed; in general the only thing left is to recalculate normals there with Ctrl+N (or with all selected).